A submarine cable is a cable installed at a bottom of a sea to transmit power between two points, e.g., between continents or between a land and an island, which are separated from each other while having a sea therebetween. FIGS. 1A and 1B are schematic cross-sectional views of submarine cables.
Generally, as illustrated in FIG. 1A, a submarine cable 1000′ may include a cable core 100′ having a conductor 110′, an inner semiconductive layer 120′ covering the conductor 110′, an insulating layer 130′ covering the inner semiconductive layer 120′, an outer semiconductive layer 140′ covering the insulating layer 130′, and a metal sheath layer 150′ covering the outer semiconductive layer 140′; and a cable protective layer 600′ covering the cable core 100′, etc. The cable protective layer 600′ may include, for example, an inner sheath 610′, a metal reinforcing layer 630′, bedding layers 620′ and 640′ provided on and below the metal reinforcing layer 630′, an outer sheath 650′, an armor 660′, and an outer serving layer 670′, etc.
Alternatively, as illustrated in FIG. 1B, a submarine cable 1000′ may include a plurality of cable cores 100′ and a cable protective layer 600′ covering the cable cores 100′. Here, each of the cable cores 100′ may include a conductor 110′, an inner semiconductive layer 120′ covering the conductor 110′, an insulating layer 130′ covering the inner semiconductive layer 120′, an outer semiconductive layer 140′ covering the insulating layer 130′, a metal sheath layer 150′ covering the outer semiconductive layer 140′, and a sheath 160′ covering the metal sheath layer 150′.
Since the submarine cable 1000′ is installed at a bottom of a sea, it is likely to be damaged by an anchor or fishing gear of a ship in a region in which a fishery activity is active or is likely to be damaged due to a natural phenomenon, such as strong sea breeze caused by ocean current or waves, or when chafed by a seabed. Generally, to prevent this problem, the submarine cable 1000′ includes the armor 660′ formed of a metal wire.
The armor 660′ is a structural reinforcing part which reinforces mechanical features and performance of the submarine cable 1000′ and provides resistance against external damage thereto during the handling and installation of the submarine cable 1000′. Generally, the armor 660′ may be formed of middle/low carbon containing steel, galvanized steel, copper, brass, bronze, or the like, and may be formed by horizontally winding wires each having a round or flat cross section.
Generally, the submarine cable 1000′ is installed in the water but a section thereof is buried into a different environment, e.g., land such as a seaside, an adjacent inland site, or an edge of a canal. An ambient temperature of the land is higher than a temperature in the water. Thus, a rated current of the submarine cable 1000′ representing current transfer capability is determined by the section of the submarine cable 1000′ buried in the land.
That is, magnetic domains in wires formed of a ferromagnetic material having high magnetic permeability such as middle/low carbon containing steel and constituting the armor 660′ are rotated due to a change in a magnetic field generated from current flowing through the conductor 100′. The rated current of the submarine cable 1000′ is additionally limited due to an increase in a temperature thereof, caused by magnetic hysteresis loss due to the rotation of the magnetic domains. A problem due to the increase in the temperature caused by the magnetic hysteresis loss is more serious at the section of the submarine cable 1000′ buried in the land of a higher ambient temperature than at the section of the submarine cable 1000′ installed at the bottom of the sea and cooled by seawater. Therefore, the rated current of the submarine cable 1000′ is limited by not only the section of the submarine cable 1000′ buried in the land but also eddy currents induced due to a conductive material of the armor 660′ and causing energy loss in the form of heat.
Thus, in a submarine cable according to the related art, wires forming a portion of an armor included in a first section installed in the water are general steel wires, and wires forming a portion of the armor included in a second section buried in the land are non-ferromagnetic metal wires substantially having no ferromagnetic property, e.g., stainless steel wires, to minimize magnetic hysteresis loss and a temperature change due to the magnetic hysteresis loss, thereby minimizing a limitation in the rated current of the cable.
However, in the submarine cable according to the related art, butt-welded parts may be particularly vulnerable to a tensile force applied to the submarine cable according to the related art and portions of the armors near the butt-welded parts may be damaged when the steel wires and the stainless steel wires forming the portions of the armor are coupled to each other at a border between the first and second sections by butt welding or the like.
Furthermore, in the submarine cable according to the related art, if the steel wires and the stainless steel wires forming the portions of the armor are coupled to each other at the border between the first section and the second section by butt welding or the like, bimetallic corrosion, i.e., galvanic corrosion, may occur and thus the armor may be damaged when the butt-welded parts and contact surfaces between the steel wires and the stainless steel wires are exposed to seawater which is an electrolyte.
As disclosed in U.S. Pat. No. 8,686,290, in the submarine cable according to the related art, a galvanic anode such as a zinc rod is bonded onto the butt-welded parts of the steel wires and the stainless steel wires in a lengthwise direction of the wires to suppress the galvanic corrosion. However, an external diameter of the cable is locally increased and the structure of the cable is unstable due to the galvanic anode protruding from the wires. Furthermore, a surface of the cable becomes irregular and thus the cable is likely to be damaged during the manufacture of the cable or when the cable passes through an installation path.
Accordingly, there is an urgent demand for a submarine cable capable of effectively suppressing damage to and corrosion of an armor formed of different types of metals due to a local decrease in tensile strength thereof, and capable of avoiding an increase in an external diameter of the cable, structural instability of the cable, and damage to the cable during the manufacture and installation of the cable.